The invention relates to a telephone interface circuit having a responding function and liable to operate steering towards various terminals, one at least of said terminals being a telephone set.
More specifically, the invention relates to such a line powered telephone interface circuit operating without an external supply source (mains or battery) but which receives all necessary electrical supply from the telephone line within the limits allowed by the telephone networks.
Telephone interface systems are known wherein a responding type apparatus, establishes a connection with the telephone line, which is equivalent to a telephone set going off the hook. A multi-frequency decoder identifies an additional dialling signal re-emitted on the line and consequently connects the telephone line to a desired terminal.
Generally, such systems couple to remote control apparatuses, memories associated with counters or microprocressors to a transmit data over a telephone line, so that the systems operate independently of the telephone sets on specific telephone lines.
However, in some cases, it is desired to selectively implement a connection either towards a specific terminal or towards a telephone set.
In such a case, a problem is that the common type telephone sets comprises an internal ring which configured for ringing in response to a ring signal transmitted by the telephone line. The connection with the line being automatic, if one wants to operate the telephone ring, once the line has been connected, it is necessary to generate a ring signal within the interface circuit to operate the telephone ringer. Typically, ring signal is .+-.48 volts. The generation of such a signal makes necessary to associate the interface circuit with an electrical supply source, for example commerical A.C. power or a battery.
According to the present invention, it is desired to provide an apparatus which is powered by the telephone line and does not require external power. Thus, even if power is not available at specific subscriber location, it is possible to have a remote survey of meters, such as the electricity meter, the gas meter, the water meter, etc.